1. Field of the Invention
The present invention relates, generally, to implantable cardioverter defibrillators (ICDs) and defibrillation methods, and particularly to a method and apparatus for determining the optimal shock strength for defibrillation, and most particularly to determining the upper limit of vulnerability (ULV) based on changes with respect to time in the T-wave of the cardiac signal, preferably the maximum of the first derivative of the T-wave with respect to time measured preferably exclusively from implanted electrodes. Unless otherwise indicated, the term “derivative of the T-wave” refers to the first derivative of the T-wave with respect to time. The technology is useful for automating the process of selecting the first defibrillation shock strength for ICDs.
2. Background Information
Heart disease is a leading cause of death in the United States. The most common form of cardiac death is sudden, caused by cardiac rhythm disturbances (arrhythmias) in the form of a ventricular tachycardia or ventricular fibrillation.
Ventricular tachycardia is an organized arrhythmia originating in the ventricles. It results in cardiac contractions that are too fast or too weak to pump blood effectively. Ventricular fibrillation is a chaotic rhythm disturbance originating in the ventricles that causes uncoordinated cardiac contractions that are incapable of pumping any blood. In both ventricular tachycardia and ventricular fibrillation, the victim will most likely die of “sudden cardiac death” if the normal cardiac rhythm is not reestablished within a few minutes.
Implantable cardioverter defibrillators (ICDs) were developed to prevent sudden cardiac death in high risk patients. In general, an ICD system consists of implanted electrodes and a pulse generator that houses implanted electrical components. The ICD uses implanted electrodes to sense cardiac electrical signals, determine the cardiac rhythm from these sensed signals, and deliver an electrical shock to the heart if life-threatening ventricular tachycardia or ventricular fibrillation is present. This shock must be of sufficient strength to defibrillate the heart by simultaneously depolarizing all or nearly all heart tissue. Shock strength is typically measured as shock energy in Joules (J). The defibrillating shock interrupts the abnormal electrical circuits of ventricular tachycardia or ventricular fibrillation, thereby permitting the patient's underlying normal rhythm to be reestablished. ICD pulse generators are implanted within the patient and connected to the heart through electrodes to provide continuous monitoring and immediate shocking when a life-threatening rhythm disturbance is detected. Because the devices must be small enough for convenient implantation, ICDs are limited in their ability to store electrical energy. In general, ventricular tachycardia can be terminated by weaker shocks than those required to terminate ventricular fibrillation. Thus ICDs must deliver a sufficiently strong shock to insure reliable defibrillation in response to each occurrence of ventricular fibrillation.
One method is to use the maximum shock strength of the ICD for each shock. However, this approach is an inefficient use of the ICD's limited stored electrical energy and will unnecessarily reduce the useful life of an ICD pulse generator.
It is well known in the art that the shock strength required to defibrillate a human heart effectively varies with the implanted lead configuration and placement as well as the individual heart's responsiveness to the shock. To maximize efficiency of an ICD system, the minimum shock strength necessary to defibrillate an individual patient's heart reliably must be determined.
However, it is also well known in the art that the relationship between an ICD's defibrillation shock strength and success or failure of defibrillation is represented by a probability-of-success curve rather than an all-or-none defibrillation threshold (DFT). Very weak, low strength (low energy) shocks never defibrillate. Very strong shocks, at energies greater than the maximum output of ICDs, always defibrillate. However, clinically relevant shock strengths for ICDs lie between these two extremes. In this intermediate range of shock strengths, a shock of a given strength may defibrillate successfully on one attempt and not on another attempt.
Determining a complete curve of the probability of success for every possible shock strength requires many fibrillation-defibrillation episodes. In clinical (human) studies and procedures, the number of fibrillation-defibrillation episodes should be limited because of their associated risks. Thus the goal of testing at the time of ICD implant cannot be to determine a complete probability of success curve. In general, the goal of testing at ICD implant is to provide an accurate estimate of the minimum shock strength that defibrillates with a high probability of success while using a minimum amount of testing. The shock energy that defibrillates with an X % probability of success is referred to as the defibrillation thresholdX or DFTX. Thus a goal of clinical testing at ICD implantation is to estimate a shock strength in the range of the DFT95-DFT99. This is the optimal strength at which to program the first shock of an ICD. For research purposes, it may be preferable to estimate the DFT50.
The minimum measured shock strength that defibrillates during a given session of defibrillation testing is referred to, in general, by the term DFT, despite the fact that no true threshold for defibrillation exists. All methods for determining the DFT of an ICD system require inducing fibrillation a number of times and testing various shock strengths for defibrillation through the implanted defibrillation leads. In the commonly used step-down method defibrillation is attempted at a high shock strength that is likely to defibrillate the heart successfully. If this shock is unsuccessful, a stronger “rescue shock” is delivered to effect defibrillation. Regardless of the outcome of the defibrillation shock, there is a waiting period of about 5 minutes to permit the patient's heart to recover. If the defibrillation shock is successful, fibrillation is reinitiated and the defibrillation is attempted at a lower shock strength. This process is repeated with successively lower defibrillation shock energies until the shock does not defibrillate the heart. The minimum shock strength that defibrillates is the DFT. Depending on the initial shock strength, the DFT determined in this manner is usually between the DFT30 and DFT70. The ICD is then programmed to a first-shock strength selected to be an estimate of the lowest value that can reliably achieve defibrillation by adding an empirically-determined safety margin to the DFT.
Other methods for determining the DFT require additional fibrillation-defibrillation episodes after a defibrillation shock has failed. In these methods, fibrillation is reinitiated after a failed defibrillation shock and defibrillation is attempted at successively higher shock strengths until a shock defibrillates the heart successfully. This change from a shock strength that does not defibrillate to one that does (or vice versa) is called a reversal of response. DFT methods may require a fixed number of reversals. If the size of the shock increments and decrements is the same, a multiple-reversal (up-down) method provides a good estimate of the DFT50. An alternative Bayesian method uses a predetermined number of unequal shock increment steps and decrement steps to estimate an arbitrary, specific point on the DFT probability of success curve.
One significant disadvantage of all DFT methods is the necessity to repeatedly fibrillate and then defibrillate the patient's heart to determine the DFT. For example, U.S. Pat. No. 5,531,770 describes a method of DFT testing that is described as an advantage because it limits the number of fibrillation-defibrillation episodes to 5, in contrast to other methods such as the “three-reversal” method that may require more episodes. These repeated episodes of fibrillation and defibrillation may have an adverse effect on the patient. Further, each fibrillation episode is associated with a small risk that the patient cannot be defibrillated and will thus die. Considerable time must be spent between test cycles in order to provide the patient's heart time to recover from the previous round of fibrillation-defibrillation.
A second disadvantage is that successful defibrillation is a probability function of shock energy, not an all or none phenomenon described by a simple threshold. Since the usual clinical DFT method results in a measurement somewhere in the broad range between the DFT30 and DFT70, optimal ICD programming cannot be achieved by adding a single empirically-determined shock increment. The resulting programmed first shock strength sometimes results in selecting a shock that either does not defibrillate reliably or unnecessarily uses excessive energy.
It is known in the art that shocks delivered during the vulnerable period of the normal cardiac cycle induce ventricular fibrillation, providing that the shock energy is greater than a minimum value and less than a maximum value. The ULV is the shock strength at or above which fibrillation is not induced when a shock is delivered during the vulnerable period of the normal cardiac cycle. The ULV may be displayed graphically as the peak of the vulnerable zone, a bounded region in a two-dimensional space defined by coupling interval (time) on the abscissa and shock strength on the ordinate. The ULV, which can be measured in regular rhythm, corresponds to a shock strength that defibrillates with a high probability of success and correlates strongly with the DFT. Because the ULV can be determined with a single fibrillation-defibrillation episode, it has the potential to provide a patient-specific measure of defibrillation efficacy that requires fewer fibrillation-defibrillation episodes than DFT testing.
Although the vulnerable period occurs generally during the T-wave of the surface electrocardiogram (ECG), its precise timing varies from individual to individual. More importantly, the peak of the vulnerable zone, which corresponds to the most vulnerable time intervals in the cardiac cycle, also varies from individual to individual. Accurate determination of the ULV depends critically delivering a T-wave shock at the peak of the vulnerable zone.
Several methods of determining the defibrillation shock strength for ICDs are based on the ULV. One such method is disclosed in U.S. Pat. No. 5,105,809. This method begins by applying an initial electrical shock to the patient's heart during the vulnerable period. The shock is timed during the “occurrence of the T-wave.” The shock strength of the initial shock is sufficiently high so as to have a low probability of initiating fibrillation. Assuming this initial shock fails to induce fibrillation, a second shock of less magnitude is delivered with the same timing during a subsequent vulnerability period. The process is repeated with successive shocks of lesser magnitudes until fibrillation is induced. When fibrillation finally occurs, the energy of the preceding shock that did not cause fibrillation is the shock strength required to defibrillate. This method does not disclose how the single shock at each energy is timed to coincide with the peak of the vulnerable zone. Indeed, it does not mention the peak or most vulnerable time in the vulnerable zone.
Another method for establishing a ULV is disclosed in U.S. Pat. No. 5,346,506. The method relies on research demonstrating that the 50% probability of successful defibrillation can be approximated by determining the 50% probability that a shock exceeds the ULV. A shock is applied to the heart through epicardial patches at a predetermined limited period of time centered on the mid-upslope of the T-wave. The disclosure argues that the total number of shocks is reduced by not having to scan the entire T-wave with shocks. A disadvantage of this method is that the shock strength for the first application must be estimated beforehand. The number of shocks required to determine the DFT is reduced only if the estimated 50% probability of reaching the ULV is quite accurate. Further, this method requires multiple fibrillation-defibrillation episodes, with their attendant risks, to provide an accurate estimate of the shock energy required to achieve a 50% probability of successful defibrillation.
U.S. Pat. No. 5,954,753 discloses that the ULV can be determined by one or two T-wave shocks timed near the peak of the T-wave, preferably about 10% of the QT interval or 30 ms before the peak.
The methods described in U.S. Pat. No. 5,105,809, U.S. Pat. No. 5,346,506, and U.S. Pat. No. 5,954,753 depend critically on accurate, a priori knowledge of the timing of the peak of the vulnerable zone because shocks are delivered at only one or two time intervals. Because the timing of this peak differs relative to any fixed point in the T-wave from patient to patient, it is not necessarily contemporaneous with any single timing interval based on the T-wave. As a result, these methods are susceptible to error because the specific time during the T-wave at which shocks are delivered may have substantially less vulnerability than peak of the vulnerable zone. If T-wave shocks are not delivered at the peak of the vulnerable zone, the ULV will be underestimated. This discrepancy will not be appreciated at the time of implantation and therefore these methods may substantially underestimate the required defibrillation shock energy setting.
Further, as will be seen in the discussion of U.S. Pat. No. 5,564,422 below, U.S. Pat. No. 5,954,753 does not identify the peak of the vulnerable zone relative to the peak of the T-wave since the interval it teaches for timing of T-wave shocks is shorter than the most vulnerable intervals for typical clinically-used transvenous ICD systems.
U.S. Pat. No. 5,564,422 to Chen and Swerdlow, which is incorporated by reference, usually provides a reliable estimate of the DFT for the clinical purpose of implanting an ICD for two-electrode transvenous defibrillation systems. However, such systems are no longer in widespread use. However, in practice, the method and apparatus disclosed in this patent has been found to require measuring and analyzing multiple surface ECG leads. The method and apparatus disclosed in this patent cannot be performed using intra-cardiac leads exclusively.
The Chen and Swerdlow method bases the timing of T-wave shocks on the latest-peaking monophasic T-wave recorded from the surface ECG. The timing of the peak of the T-wave varies substantially among ECG leads as a consequence of QT-interval dispersion. In different patients, the latest-peaking monophasic T-wave occurs unpredictably in various surface ECG leads. Even a small error in measuring the pacer spike to peak interval of the latest-peaking monophasic T-wave can result in errors in the measured ULV and thus compromise its value as a clinical tool. Thus, to be used accurately, this method requires measurement and analysis of intervals from multiple (preferably all 12) standard surface ECG leads to identify the lead with the latest-peaking monophasic T-wave.
The Chen and Swerdlow method cannot be performed using ECGs recorded exclusively from implanted electrodes (electrograms). Implanted electrodes, particularly those including intra-cardiac electrodes, often have biphasic rather than monophasic T-waves as shown in FIG. 3. When this occurs, the peak of the (monophasic) T-wave is undefined and this method cannot be applied. Further, even if an intra-cardiac electrode has a monophasic T-wave, the peak may precede that of a surface ECG lead.
This limitation also applies to the method of U.S. Pat. No. 5,954,753 which depends on identification of the peak of the T-wave. This method recommends identification of the peak of T-waves recorded from ICD electrograms, which usually have biphasic T-waves. FIG. 3A shows that the peak of the latest-peaking monophasic T-wave on the surface ECG agrees closely with the peak of the derivative of the intracardiac electrogram, but not with the peak of the biphasic intracardiac electrogram. Further, as FIG. 3A shows, the T-wave recorded from an intracardiac electrogram often is low in amplitude. Thus identification of its peak may be subject to significant measurement error.
The present invention differs from the Chen and Swerdlow (U.S. Pat. No. 5,564,422) method in several respects, including but not limited to, that the coupling interval of T-wave shocks is based on the point of maximum derivative of the repolarization phase (T-wave) of an electrogram recorded from an implanted electrode, whether the T-wave is biphasic or monophasic. This approach involves the concept of the activation-recovery interval. The activation-recovery interval is the interval between the times of minimum derivative of the ventricular electrogram and maximum derivative of the T-wave in a unipolar intra-cardiac electrogram. Theoretical analysis predicts that maximum derivative of the T-wave is proportional to a spatial weighting function of the third temporal derivative of the cardiac action potential. Because the maximum of the first temporal derivative of the action potential times very closely with the maximum of its third temporal derivative, the activation-recovery interval has been used as a measure of local repolarization in basic physiologic studies. Because the activation-recovery interval acts as a spatial average, it is dominated by the action potentials of cells closest to the recording site. The activation-recovery interval recorded from a point, intra-cardiac electrode has been used to assess local repolarization. For example, it has been used to assess dispersion of local repolarization intervals in canines and local effects of catheter ablation in humans.
In an embodiment of the present invention, the analyzed electrogram may be recorded from large extra-cardiac or intra-cardiac electrodes. Recordings from these large electrodes contain more information regarding global repolarization than recordings from point electrodes.
An additional limitation of the method of Chen and Swerdlow (U.S. Pat. No. 5,564,422) is that the timing of the latest peaking T-wave is measured only once at the beginning of testing. Research has shown that the interval between the pacer spike and the peak of the latest peaking T-wave may change over time during the testing procedure as a result of shock delivery. Since the method of Chen and Swerdlow (U.S. Pat. No. 5,564,422) sets the timing of all subsequent shocks by using the initial measurement of the interval between the pacer spike and peak of the latest-peaking monophasic T-wave, subsequent shocks may not be delivered at the desired time relative to the peak of the latest peaking T-wave at the moment the shock is delivered. Even a small error in measuring the pacer spike to peak interval of the latest-peaking monophasic T-wave (of the order introduced by post-shock changes that occur during clinical testing after several shocks) can result in errors in the measured ULV and thus compromise its value as a clinical tool. (Swerdlow C D, Martin D J, Kass R M, Davie S, Mandel W J, Gang E S, Chen P S. The zone of vulnerability to T-wave shocks in humans. J Cardiovasc Electrophysiol. 1997; 8:145-54.) An embodiment of the method of the present invention avoids such errors by re-measuring the pacer spike to peak interval of the derivative of the intra-cardiac T-wave automatically after each shock.
A further limitation of the method of Chen and Swerdlow (U.S. Pat. No. 5,564,422) is that it does not scan the vulnerable zone completely for the defibrillation electrode configuration most commonly used in clinical practice today. This may lead to underestimation of the ULV in some patients, which in turn may lead to programming of insufficient first ICD shock strengths and failed defibrillations. The method of Chen and Swerdlow usually provides an adequate scan of the vulnerable zone when shocks are delivered using a two-electrode system from right-ventricular coil to left-pectoral ICD case (also referred to as the “housing” or “can”). (Swerdlow C D, Martin D J, Kass R M, Davie S, Mandel W J, Gang E S, and Chen P S, in The Zone of Vulnerability to T-wave shocks in Humans, J Cardiovasc Electrophysiol. 1997; 8: 145-54). However, in present ICDs, the principal shock pathway uses a different three-electrode system to deliver shocks from right-ventricular coil to left-pectoral case plus a superior vena cava electrode. It provides superior defibrillation to the two-electrode system. Recently the inventor has demonstrated that the peak of the vulnerable zone using this three-electrode defibrillation configuration may not be at any of the intervals tested in the method of Chen and Swerdlow (U.S. Pat. No. 5,564,422) or those tested in the method of U.S. Pat. No. 5,954,753. Instead the most vulnerable intervals time after the peak of the latest peaking T-wave in some patients. In fact, in 5 of 25 patients tested (20%), the peak of the vulnerable zone occurred at least 20 ms beyond the peak of the latest peaking T-wave. In these patients, the methods of U.S. Pat. No. 5,564,422 and U.S. Pat. No. 5,954,753 do not accurately identified the ULV.
In one embodiment of the present invention, four shocks are used to scan the vulnerable zone reliably in humans. When the surface ECG is used as a reference, these shocks should be delivered at −40 ms, −20 ms, 0 ms, and +20 ms relative to the peak of the latest-peaking monophasic T-wave. Clinical application of this four-shock method with T-wave shocks timed relative to the surface ECG has resulted in programming of shock strengths that defibrillate with near uniform success for both two-electrode and three-electrode defibrillation configurations. Research has demonstrated that the shock strength equal to the step-down ULV determined by the present invention, successfully defibrillates 90% of the time with a 95% confidence level of plus or minus 8%. When the shock strength is increased to a value that is 3 Joules above the measured ULV, the rate of successful defibrillation is 100% with a confidence level greater than 95%.
For the present, standard three-electrode right-ventricular coil to left-pectoral case plus a superior vena cava shock pathway, three shocks at −20 ms, 0 ms, and +20 ms relative to the peak of the latest-peaking monophasic T-wave correctly identified the most vulnerable intervals in all 25 of 25 patients tested (100%). In the same patients, if shocks were timed relative to the maximum of the derivative of the T-wave, four shocks timed at −20 ms, 0 ms, and +20 ms, and +40 ms were required to identify the most vulnerable intervals in 24 of 25 patients (96%). In the remaining patient, the T-wave scan missed the most vulnerable interval by 9 ms. Thus, for the three-electrode shock pathway an alternative embodiment delivers up to three test shocks at −20 ms, 0 ms, and +20 ms relative to the peak of the latest-peaking monophasic surface T-wave. A second alternative embodiment for this three-electrode shock pathways delivers up to four test shocks at −20 ms, 0 ms, and +20 ms, and +40 ms relative to the maximum of the derivative of the T-wave of the ICD electrogram.
All US patents and patent applications, and all other published documents mentioned anywhere in this application are incorporated by reference in their entirety.